This invention relates to metal detection apparatus.
Numerous metal detection circuits have been proposed. Most such circuits employ an oscillator circuit including an inductive detection coil. The presence of metal near the coil alters the parameters of the oscillating circuit, and the resulting variation in the output of the oscillating circuit is detected. Circuits of the kind described are sensitive to various electromagnetic signals, including radio frequency and mains-borne disturbances. Such circuits are also frequently highly sensitive to temperature variations and to variations in component values. This may result in unsatisfactory performance, or may necessitate the use of complex and expensive compensating circuitry, and the selection of expensive, high quality components.
It is an object of the invention to provide a metal detection circuit which is relatively insensitive to temperature variations and to electromagnetic interference.